Process for completing wells in unconsolidated formations by reverse in situ combustion



May 11, 1965 R. L. REED 3,182,722

PROCESS FOR COMPLETING WELLS IN UNCONSOLIDATED FORMATIONS BY REVERSE IN SITU COMBUSTION Filed Dec. 19, 1961 3 Sheets-Sheet 1 IN V EN TOR.

xP0/1/4L0 L. PEA D BY rraevev y 1955 R. 1.. REED 3,182,722

PROCESS FOR COMPLETING WELLS IN UNCONSOLIDATED FORMATIONS BY REVERSE IN SITU COMBUSTION Filed Dec. 19, 1961 3 Sheets-Sheet 2,

INVENTOR. Pat 4Z0 L. AAZE BY May 11, 1965 R. L. REED ETING WELLS IN UNCONSOLIDAT REVERSE IN SITU COMBUSTION 3 Sheets-Sheet 3 PROCESS FOR COMPL FORMATIONS BY Filed Dec. 19, 1961 Wyn me. Ad/VdLD 4. 2550 Arrange-V United States Patent 3,182,722 PROCESS FDR COMPLETING WELLS IN UNCON- SOLIDATED FORMATIONS BY REVERSE IN SITU COMBUSTION Ronald L. Reed, Allison Park, Pa., assignor to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Dec. 19, 1961, Ser. No. 160,494 3 Claims. (Cl. 166-45) This invention relates to the completion of wells for producing fluids from unconsolidated formations. It is more particularly concerned with the creation of strong coherent permeable walls of a borehole capable of preventing movement of individual particles of the unconsolidated formation into a well during production of well Many subsurface formations containing oil or gas consist of loosely cemented particles of sand, During production of fluids from the well, sand particles flow into the well and cover perforations in the casing. Rates of production of fluids from the well are thereby reduced. Situations have been encountered where the subsurface formations were so friable that sand flowed from the formation into the well fast enough to cover perforations in the casing in less time than is required to pull wash pipe used to clean the Well from the well and run production tubing into the well. In addition to the difficulty with the sand plugging the well and causing reduction in the rate of Well fluids, the flow of sands from the formation into the well sometimes results in shifting of the formation which damages the well casing. If the subsurface formation contains fluids under high pressure, the sand may fiow into the well and through production equipment at high rates causing erosion which severely darnages the equipment.

One method of combating the production of sand from unconsolidated formations has been to install slotted liners in the borehole through the interval of the unconsolidated formation. The slots in the liners are small enough to prevent flow of sands from the formation into the well. Slotted liners are frequently unsatisfactory because the movement of sands from the formation into contact with the liner results in plugging of the slots or collapse of the liners.

It has also been proposed to bond the particles of unconsolidated formations together by supplying heat to the formation by means of electric heaters or borehole burners to raise the temperature of the formation surrounding the borehole to a temperature at which oil in the formation Will become coked and bond particles together. Such techniques have generally been unsuccessful. -Bore hole burners usually require an excess of oxygen for satisfactory operation. If oxygen is present in the injected gas, forward burning will generally be initiated. The forward burning removes substantially all of the carbonaceous material from that portion of the pay zone traversed by the combustion front. Apparatus for produc tion of an inert gas usually is not available at a Well site.

This invention resides in a method for the formation of a strong coherent permeable mass at the borehole wall of a well in which reverse combustion is caused to proceed vertically around the borehole of a well across the interval of the unconsolidated formation through sands containing oil to form a cylinder of coke surrounding the borehole. In a preferred form of this invention, the desired strong coherent permeable mass is prepared by packing the borehole with sand and a cokable oil around air supply tubing extending down the borehole and terminating below the lower boundary of the unconsolidated formation. Thereafter, an oxygen-containing gas is passed through the resultant sand pack, and ignition of oil in the ice and the tubing is removed. If necessary to enlarge the hole extending longitudinally through the sand pack, a hole of the desired diameter may be drilled through the coked mass. It has been found that the coke remaining in the sand pack after the reverse combustion front has traversed the length of the sand pack is strong, permeable, and highly resistant to dissolution by oil.

In the drawings:

FIGURE 1 is a diagrammatic illustration, partially in vertical section, of a well having casing extending through the unconsolidated formation and in which the coked mass is formed around the casing;

FIGURE 2 is a diagrammatic view of a well, partially in vertical section, in which the borehole of the well is underreamed to an enlarged diameter and packed with sand and oil for the creation of a mass of sand particles bonded together with coke;

FIGURE 3 is a diagrammatic illustration, partially in vertical section, of the well illustrated in FIGURE 2 after creation of the mass of sand particles bonded with coke and drilling a central borehole through the resultant mass for reception of production tubing; and

FIGURE 4 is a diagrammatic view, partially in vertical section, of a well, similar to that illustrated in FIGURE 1, adapted for discharging combustion gases into a deeper formation;

FIGURE 5 is a diagrammatic view, partially in vertical section, of the well illustrated in FIGURE 2 modified for discharging combustion gases into a deeper formation.

Referring to FIGURE 1, a well indicated generally by reference numeral 10 is drilled through an unconsolidated formation 12 constituting the pay zone from which fluids are to be produced to a total depth -14 in base rock 16 underlying pay zone 12. Above the pay zone 12 is cap rock 18. Casing '20 is run into the well through the pay zone 12 into the base rock :16 and is cemented in place in the conventional manner by the displacement of cement from the lower end of the casing upwardly through the annulus between the outer wall of the casing and the wall of the borehole to form a cement sheath 22. Pressure slightly in excess of the formation pressure of the pay zone should be maintained in the borehole at the pay zone interval during drilling and running of the casing to prevent collapse of the borehole walls through the pay zone. Stripping of oil from the pay zone adjacent the borehole can be avoided by using a crude or residual oil to maintain the desired pressure in the borehole.

Casing 20 and cement sheath 22 are perforated by conventional means such as shaped charges at 24 near the lower boundary of pay zone 12 and at 26 near the upper boundary of the pay zone. Air supply tubing 28 is run down the well to a position with its open lower end at approximately the same depth as perforations 24; A packer 30' adapted to close the annulus 32 between tubing 28 and casing 20 is run into the well on the tubing 28 and is set between the perforations 24 and 26. In those instances Where greater capacity for flow into the well is desired, casing 20 may be perforated as desired between perforations 24 and perforations 26. The perforations between perforations 24 and perforations 26 can then be isolated during the reverse burning by setting a packer immediately below perforations 26 in addition to packer 30. A thermocouple 34 is illustrated adjacent the casing 20 slightly above perforations 24. In the particular apparatus illustrated in the drawings, a lead line 36 from the thermocouple 34 extends into the tubing 28 and upwardly through the tubing to the well head where it is connected to suitable apparatus for indicating the temperature measured by the thermocouple.

A gas burner 38 is suspended by means of fuel supply tubing 4!) in the annulus 32 adjacent perforations 26 to heat oil in the pay zone 12. A burner air supply line 42 extends downwardly through tubing 40 to deliver air to the burner 38. The air is mixed in the burner with fuel gas supplied to tubing 40 through a side inlet 44. The burner 38 may, for example, be of the type described in Patent No. 2,668,519 of J. J. Piros.

Other means can be used to heat the formation adjacent the perforations 26 to a temperature at which oil present in the formation will ignite upon contact with air. For example, an electric heater may be installed in the well adjacent the perforations. Another method that can be used is to displace a pyrophoric liquid through perforations 26 into the formation.

For convenience in describing this invention, it will be assumed that pay zone 12 is a tar sand containing a heavy crude oil. This invention is not limited to use in such formations. It may be used, for example, in formations devoid of heavy hydrocarbon oils by displacing a cokable oil into the formation surrounding the borehole before initiating reverse combustion. The coking of oil in the pay zone 12 surrounding the well to cohsolidate the sands into a permeable coherent mass preventing movement of sands into the well is accomplished by displacing air down tubing 28 and through perforations 24 into the pay zone. Either simultaneously with, or before, the injection of air into the formation, air and lease gas are displaced through the respective supply lines to the burner 38 and the fuel gas is ignited at the burner. Burning in the borehole at burner 38 is continued to heat the formation adjacent the perforations to a temperature at which oil in the pay zone will ignite upon contact with the air injected into the formation through perforations 24. Products of combustion are discharged from the borehole through the vent line 46 at the well head.

Arrival of air injected into the pay zone through perforations 24 at the perforations 26 will be indicated at the well head by changes in volume and composition of the gases discharged at vent 46. If ignition occurs immediately upon arrival of the injected air at the heated formation adjacent the perforations 26, it will be indicated by the increase in volume of the gases discharged from the vent without an increase in the oxygen concentration. Once ignition has occurred, burning of fuel in burner 33 is stopped.

Ignition of oil in the pay zone adjacent the perfora tions 26 will initiate a reverse combustion process in the pay zone 12 with the combustion zone moving downwardly around the casing countercurrent to the direction of flow of the air displaced into the pay zone 12 at perforations 24. The reverse combustion zone leaves behind it a strong coherent monolithic mass indicated by dashed line 48 of particles of the pay zone bonded by a coke highly insoluble in hydrocarbons. The products of the reverse combustion flowing through the pay zone to the perforations 2e insure a high permeability in the pay zone between the combustion front and the perforations 26.

Displacement of air down the supply tubing 28 and through perforations 24 is continued until the reverse combustion front reaches the perforations 24. It is a characteristic of reverse combustion in oil-bearing zones that once the combustion front has reached the area at which an oxygen-containing gas is injected into the formation, the direction of movement of the combustion front is reversed and forward burning then occurs with the combustion front feeding on the coke that has been formed in the pay zone and moving toward the area at which combustion products are discharged from the pay zone. Such forward burning will result in destruction of the coke and must be avoided.

Reverse combustion is also characterized by a very steep temperature gradient on the upstream side of the combustion front. The thermocouple 34 will indicate the arrival of the combustion front at the lower end of the pay zone adjacent the perforations 24 by a sharp increase in temperature. When the combustion front has traversed the pay zone, displacement of oxygen-containing gas down tubing 28 is halted.

The reverse combustion can also be made to proceed from the bottom to the top of the pay zone. Air will then be displaced down the annulus 32 and products of combustion carried to the well head through tubing 28. Ignition is obtained by means of an electric heater at the bottom of the pay zone.

An important advantage of the process of this invention in comparison with other processes suggested for forming coke in 'situ to bond the particles of a friable formation is the insolubility in hydrocarbons of coke formed in a reverse combustion process. A series of tests was performed in a combustion tube packed with sand which was then saturated with crude oil from either the Santiago or Woodward fields in California. Air was introduced into one end of the tube at a measured rate and combustion initiated at the discharge end of the tube. The air flux varied over a five-fold range in the several runs to give a peak temperature measured at the wall of the combustion tube ranging from 490 F. to 950 F. The displacement of air through the tube was continued until the reverse combustion front had moved through the tube to the air inlet end. In all of the several reverse combustion runs, the coke formed was insoluble in alcohol and benzene. Examination of the cores showed them to be hard and strong.

Frequently, unconsolidated pay zones will contain shale or clay streaks which are substantially impermeable and will interfere sufficiently with vertical flow through the pay zone surrounding the borehole of the well to prevent eifective consolidation of the pay zone in the manner described with reference to FIGURE 1 of the drawings. The irregularities in the formation result in nonuniform flow rates with resultant different rates of movement of the combustion front. The different rates of flow of air through the formation also cause a wide variation both in the peak temperature of the combustion front and prevent control of optimum conditions for the production of a strong highly coherent and oil-resistant coke bonding the particles together. A preferred method of this invention which eliminates the difficulties resulting from such shale and clay streaks is illustrated in FIG- URES 2 and 3 of the drawings.

Referring to FIGURE 2, a well indicated generally by reference numeral 50 is illustrated with casing 52 extending down the well to a position with its lower end 54 in cap rock 56 overlying an unconsolidated formation 58 constituting the pay zone. The casing 52 is cemented in place in accordance with the usual procedures whereby a cement sheath 60 surrounds the casing. After the casing has been set, the drilling is continued, preferably using a heavy oil as a drilling fluid, to a total depth 62 in base rock 63 underlying the pay zone 58. To increase the thickness of the coke wall that is formed in this embodiment of the invention, the borehole is underreamed below the lower end 54 of the casing to a diameter substantially larger than the diam eter of the casing 60.

After underreaming the borehole, an air distributor 64 is run to the bottom of the well on air supply tubing 66 while pressure is maintained on the borehole to support the borehole walls through the pay zone. The air distributor '64 is secured to the lower end of the air supply tubing 66 by any suitable means, such as a shear pin 68, which will allow the tubing 66 to be withdrawn from the well after the coking operation.

A mixture of sand and a cokable oil such as a crude or residual oil is then packed in the underreamed portion of the borehole surrounding the tubing 66 to form a sand pack 70. By continuing the drilling into the base rock the sand pack can extend below the pay zone 53 to permit formation of a coked mass extending completely through the interval of the pay zone. It is desirable that the sand saturated'with crude oil be firmly and uniformly packed against the wall of the borehole by a suitable tamping arrangement which may, for example, be opearted by a wire line. If desired, the sand can be packed in the well in a dry condition, and thereafter saturated with oil displaced down the well and squeezed into the sand pack. Such squeezing aids in forcing the sand pack against the pay zone as well as in consolidating the sand pack. If dry sand is used in forming the sand pack, pressure should be maintained on the hole with a gas.

It is important to the production of a strong bonded mass that sand particles having a wide range of particle size be used in sand pack 70. A substantial fraction of the sand particles should fall within a range of 80 to 140 mesh to increase the area of contact between individual particles and reduce the size of the openings. The larger particles in the sand pack may be any desired size. Particles smaller than 140 mesh tend to plug the mass and should be eliminated from the sand pack. Suitable sand for packing in the borehole of the well '50 can be obtained by crushing Berea sandstone. Tar sands such as the Athabasca tar sands from which any clay and shale which may have been present have been removed are also suitable. A layer 72 of coarse sand or gravel is placed on the upper surface of the sand pack 70 to prevent entrainment of sand in the gases discharged from the sand pack.

When the sand pack is formed by placing dry sand in position around tubing 66 followed by squeezing oil into the sand pack, it may be desirable to remove excessive amounts of oil from the sand pack. This may be accomplished by displacing air through the sand pack to carry oil upwardly into the lower end of the casing above the layer 72 with gravel. The oil thus displaced from the sand pack may be removed from the well, for example, by bailing. It is usually desirable that the oil in the sand pack be present in a concentration of the order of 8 to 9 percent by weight of the oil and sand mixture, although concentrations ranging from 5 to about 20 percent may be effective. If the oil used in the sand pack is a heavy residual oil, smaller amounts can be used because of the greater amount of coke produced by such heavy oils. Coke constituting approximately 1 to 10 percent by weight of the bonded mass has been found to produce structure of adequate strength and permeability.

In the embodiment of the invention illustrated in FIG- URE 2, ignition of the oil in the sand pack is accomplished by delivering a pyrophoric liquid such as triethylborane down the well and into the layer '72 of gravel. Thereafter air is displaced through tubing 66 and distributor 64 into the sand pack. As the air passes upwardly through the gravel 72 and comes in contact with pyrophoric liquid, the pyrophoric liquid is ignited to heat oil in the top of the sand pack 70 to a temperature at which it is ignited. Combustion products are discharged from the Well through vent 74.

A preferred method of making the reverse combustion front move upwardly through the sand pack is to continue the borehole of the well to a lower permeable sand. The permeability of the lower sand may be caused by fractures. The sand pack is formed and air is injected into the top of the sand pack, ignition is commenced near the bottom of the sand pack, and the products of combustion are displaced down the well into the lower permeable sand. The same technique of disposing of the combustion products in lower sands also can be used advantageously in the embodiment of the invention illustrated in FIGURE 1. In this technique, running of air supply tubing down the well is not required.

Referring to FIGURE 4, a well similar to that illustrated in FIGURE 1 is shown modified for discharging the products of combustion into a permeable formation '78 below the pay zone 12. If desired, the formation 78 may be fractured as indicated by reference numeral 80.

The unconsolidated formation 12 is heated adjacent the lower perforations 24 by any of the previously mentioned means, and thereafter a plug 82 is set in the well in place of the packer 30. Air is injected into the upper end of the well through a nozzle 84 extending through the cap closing the upper end of the well. The air passes outwardly through perforations 26 and flows downwardly through the unconsolidated formation 12. When the injected air encounters the heated zone adjacent the perforations 24, reverse combustion commences and the com bustion zone moves upwardly toward perforations 26. Combustion products are discharged from the unconsolidated formation 12 into the well through perforations 24 and are displaced in the formation 78.

In the embodiment of the invention illustrated in FIG- URE 5, the borehole of the well is extended downwardly through base rock 63 into a lower permeable formation 86. Formation 86 is illustrated with a fracture 88 extending into it from the well. Air is injected downwardly through the well into the upper end of the sand pack 70. The sand pack is ignited near its lower end and reverse combustion proceeds upwardly through the sand pack. Combustion products discharged from the lower end of the sand pack are displaced downwardly into formation 86.

It is one of the important advantages of this invention that the uniform nature of the sand pack 70 permits accurate control of the temperature in the sand pack to form coke of optimum properties. It has been found that the peak temperature in the combustion front can be controlled directly by the air flux. A peak temperature of 450 F. is adequate to cause coking. Temperatures in excess of 1300 F. should be avoided because such temperatures will leave areas of clean sand within the sand pack. It is preferred that the air flux be adjusted to maintain a peak temperature between about 600 F. and 1000 F. to produce a strong, oil-resistant coke. An air flux in the range of about 10 to (standard cubic feet/hour/ square foot) will produce such temperatures in a substantially adiabatic system such as exists in a well. It is expected that a portion of the air injected into the sand pack may flow into the pay zone 58 adjacent to the sand pack and flow upwardly through the pay zone. Such flow and the resultant coke formation are advantageous in bonding the monolithic mass of the sand pack firmly to the pay zone. The amount of air flowing upwardly through the pay zone will ordinarily be small, particularly if the pay zone has a number of shale streaks running through it.

Displacement of air through the sand pack is continued until the temperature at the lower end of the sand pack, as indicated by the thermocouple 76, indicates that the combustion front has traversed the full length of the sand pack. Injection of air into the well is then stopped and tubing 66 is pulled from the well. Because of its detachable connection to the lower end of the tubing 66, the air distributor 64 remains in the well.

After tubing 66 has been pulled from the well, a drill is run downwardly through casing 52 to drill a borehole of the desired diameter, as illustrated in FIGURE 3, through the bonded sand pack 70. Production tubing 78 is then run down the Well to the interval of the pay zone 58 for delivery of fluids produced from the pay zone 58 through the permeable sand pack 70 into the borehole of the Well.

The embodiment of the invention illustrated in FIG- URES 2 and 3 is advantageous when a borehole of relatively large diameter is desired through the monolithic mass of coke-bonded sand particles. In some instances, a borehole of small diameter will be adequate for production of fluids from the pay zone. In such situations, underreaming, as illustrated in FIGURE 2, may not be necessary. Because of the high strength of coke produced by reverse combustion, coked measses three inches or more in thickness are sufficient to support the formation. The sand pack will then be made in the manner described for the embodiment of FIGURE 2 and after the reverse combustion, tubing 66 can be removed and replaced with perforated tubing for production of Well fluids. It is also contemplated that in some instances, the casing will be set entirely through the pay zone and perforated in the usual manner before forming the sand pack within the casing through the pay zone. Thereafter, the coke is formed in the sand pack by reverse combustion in the manner described for the embodiment illustrated in FIG- URE 2.

The coke formed by the reverse combustion process of this invention is particularly suitable for consolidating friable formations because the coke has greater strength than coke formed by passing air through a permeable oilbearing formation without initiation of either forward or reverse combustion. Additionally, the process of this invention does not run the danger that exists in the aircoking processes of initiating forward combustion with resultant destruction of the coke. The downward movement of the combustion zone permits accurate determination of the progress of the combustion zone, and thereby allows the injection of air to be stopped after the combustion zone has traversed the unconsolidated formation completely and before burning any appreciable amount of the coke. This invention, moreover, does not require an inert gas generator, as do processes using hot inert gases to form the coke.

I claim:

1. A process for completing a well in an unconsolidated formation penetrated by the well comprising drilling the borehole of the Well completely through the unconsolidated formation into a permeable formation below the unconsolidated formation, passing an oxygen-containing gas down the well and at the upper boundary of the unconsolidated formation into sands containing a cokable oil and downwardly through said sands, said sands extending through the interval of the unconsolidated formation, initiating reverse combustion in the sands at the lower boundary of the unconsolidated formation, continuing the displacement of air down the well and into the upper portion of the sands containing the cokable oil whereby a reverse combustion zone moves vertically through the sands to the place of injection of oxygen-containing gas into the sands terminating the injection of the oxygenoontaining gas when the combustion zone reaches the vicinity of the upper boundary of the unconsolidated formation, discharging products of combustion from the saids containing the cokable oil into the borehole of the well, and displacing the products of said combustion into the permeable formation below the unconsolidated formation.

2. A process for completing a well in an unconsolidated formation penetrated by the well, said unconsolidated formation containing a cokable oil, comprising drilling the borehole of the well completely through the unconsolidated formation into a permeable formation below the unconsolidated formation, setting casing in the borehole through the unconsolidated formation, perforating the casing in the interval of the unconsolidated formation adjacent the lower boundary thereof and also adjacent the upper boundary thereof, injecting air down the well into the unconsolidated formation near the upper boundary thereof, igniting the unconsolidated formation near the lower boundary thereof whereby a reverse combustion zone moves upwardly through the unconsolidated formation to deposit col-re bonding the formation into a strong permeable mass, terminating the injection of air when the reverse combustion zone reaches the upper boundary of the unconsolidated formation, and discharging products of combustion from the unconsolidated formation through the perforations adjacent the lower boundary thereof and into the permeable formation below the unconsolidated formation.

3. A process for completing a well in an unconsolidated formation penetrated by the well comprising drilling the borehole of the well completely through the unconsolidated formation into a permeable formation below the unconsolidated formation, forming a sand pack of sand and a cokable oil in the borehole of the well throughout the interval of the unconsolidated formation, displacing air downwardly through the well into the upper end of the sand pack, igniting oil at the lower end of the sand pack whereby a reverse combustion zone passes upwardly through the sand pack, discharging products of combustion from the lower end of the sand pack into the permeable formation below the unconsolidated formation, and terminating the injection of air When the combustion zone reaches the upper end of the sand pack.

References Cited by the Examiner UNITED STATES PATENTS 2,771,952 11/56 Simm 166-12 3,003,555 10/61 Freeman et al. 166-39 X 3,044,546 7/62 Dixon 166-25 X BENJAMIN HERSH, Primary Examiner.

CHARLES E. OCONNELL, Examiner. 

3. A PROCESS FOR COMPLETING A WELL IN AN UNCONSOLIDATED FORMATION PENETRATED BY THE WELL COMPRISING DRILLING THE BOREHOLE OF THE WELL COMPLETELY THROUGH THE UNCONSOLIDATED FORMATION INTO A PERMEABLE FORMATION BELOW THE UNCONSOLIDATED FORMATION, FORMING A SAND PACK OF SAND AND A COKABLE OIL IN THE BOREHOLE OF THE WELL THROUGHOUT THE INTERVAL OF THE UNCONSOLIDATED FORMATION, DISPLACING AIR DOWNWARDLY THROUGH THE WELL INTO THE UPPER END OF THE SAND PACK, IGNITING OIL AT THE LOWER END OF THE SAND PACK WHEREBY A REVERSE COMBUSTION ZONE PASSES UPWARDLY THROUGH THE SAND PACK, DISCHARGING PRODUCTS OF COMBUSTION FROM THE LOWER END OF THE SAND PACK INTO THER PERMEABLE FORMATION BELOW THE UNCONSOLIDATED FORMATION, AND TERMINATING THE INJECTION OF AIR WHEN THE COMBUSTION ZONE REACHES THE UPPER END OF THE SAND PACK. 